1. Field of the Invention
The present invention relates to a thermal flowmeter for measuring a flow rate with use of resistors (hot wires). More particularly, the present invention is concerned with a thermal flowmeter which can measure the flow rate of a fluid under measurement in either direction with a high accuracy.
2. Description of Related Art
Heretofore, vacuum suction has been used for handling a semiconductor chip at the time of mounting the chip, and a sucked state of the chip by vacuum suction has been confirmed using a pressure sensor. Recently, however, semiconductor chips have been becoming more and more small in size. Therefore, for example in the case of a 0.5 mm square chip, there is used a suction orifice (nozzle) having a diameter of 0.5 mm or 0.3 mm. Consequently, as shown in FIG. 30, there occurs little difference in the internal pressure of the orifice between the time when suction is performed and the time when suction is not performed. Thus, with the pressure sensor, it has become impossible to confirm a sucked state of a semiconductor chip. Under the circumstances, it has been proposed to confirm a sucked state of a semiconductor chip by detecting a flow rate of air flowing through the orifice. FIG. 30 shows an output example of a pressure sensor under the conditions of a nozzle diameter being 0.3 mm and a vacuum pressure being −70 kPa.
In view of the above-mentioned point the applicant in the present case has proposed in Japanese Patent Application No. 2000-368801 a thermal flowmeter suitable for the confirmation of suction. FIG. 31 shows the results of having made confirmation of suction under the above conditions. As is apparent from FIG. 31, it is seen that the confirmation of suction which has so far been difficult with use of a pressure sensor can be done by using the thermal flowmeter proposed in the above patent application.
However, with the thermal flowmeter which the applicant in the present case proposed in Japanese Patent Application No. 2000-368801, as shown in FIG. 32, the pressure characteristic is deteriorated as the degree of vacuum increases, and there has been a possibility that the confirmation of suction may not be done with a high accuracy. Moreover, the output characteristic of the thermal flowmeter in question is as shown in FIG. 33 in which a flowing direction cannot be detected because the same value is outputted irrespective of in which direction a fluid under measurement flows. Thus, when the flow rate in either direction cannot be detected, it is impossible to confirm release from suction although it is possible to confirm suction. This is because the fluid flowing direction when a semiconductor chip is chucked and that when the semiconductor chip is released are opposite to each other. FIGS. 32 and 33 illustrate outputs, assuming that a full-scale flow rate is 1 L/min in both cases.
For confirming suction and release by use of a flowmeter, it is necessary to use a flowmeter able to detect a flow rate in either direction. For example, such a flowmeter is disclosed in Japanese Unexamined Patent Publication No. 2002-5717. However, this flowmeter involves the problem that its output characteristic is not linear, as shown in FIG. 34. In the case of such a non-linear output characteristic, it has so far been impossible to control clogging of a nozzle. A linear output characteristic can be obtained by using an arithmetic circuit, as described for example in Japanese Unexamined Patent Publication No. 2001-165734. However, such an arithmetic circuit must be provided separately, which is disadvantageous in point of cost.
Further, the flowmeter described in Japanese Unexamined Patent Publication No. 2002-5717 involves the problem that the output becomes unstable due to the influence of a turbulent flow, as shown in FIG. 35. Once the output becomes unstable, it is required that a threshold value in the confirmation of suction be set rather low. In confirming suction, however, a very small change in flow rate is detected, so if the threshold value is set low, it is determined that suction is performed in normal condition even when suction is not in normal condition and the flow rate is smaller than that in normal suction. That is, it has been impossible to make the confirmation of suction with a high accuracy. Likewise, in case of using a collet type nozzle which chucks a semiconductor chip while ensuring a constant leakage quantity at all times, it has been impossible to make the confirmation of suction. Such fluctuations in output can be eliminated by using an electric filter, but the use of an electric filter is not desirable because the response characteristic is impaired.